Plasma high density lipoproteins (HDL) are a major protective factor in atherosclerosis. The mechanisms of the protective effect of HDL are incompletely understood. Mutations in ABCA1 result in low HDL and impaired efflux of cholesterol from macrophage foam cells to lipid-poor apoA-l. However, while ABCA1 plays a key role in HDL formation, ABCA1 interacts poorly with HDL particles that form the bulk of plasma HDL. This observation led to our recent discovery that two LXR-induced half-transporters, ABCG1 and ABCG4, mediate cholesterol efflux to HDL particles but not to lipid-poor apoA-l. ABCG1 is primarily responsible for cholesterol efflux to HDL in LXR-activated macrophages. The primary hypothesis of this project is that the activity of ABCG1 mediates the protective effect of HDL by promoting efflux of cholesterol from macrophages and possibly other vascular cells to HDL. A secondary hypothesis is that changes in cellular cholesterol efflux or distribution mediated by ABCG1 inhibit cholesterol-induced cell death, an idea we will explore collaboratively with Dr.Tabas. In Aim 1, we will evaluate the cellular mechanisms of cholesterol efflux mediated by ABCG1 and the role of ABC transporters in preventing cholesterol-induced apoptosis. In Aim 2 the role of ABCG1 in vivo will be evaluated in gene knock-out mice and effects on atherosclerosis will be determined in atherosclerosis-susceptible backgrounds. In Aim 3, the function of ABCG1 in macrophages and hepatocytes will be evaluated by development of conditional knock-outs. The specific roles of ABCG1 and ABCA1 in HDL protection will be evaluated by seeing if the protective effect of increased HDL levels resulting from apoA-l overexpression is reduced in a setting of macrophage ABCG1 and/or ABCA1 deficiency. The results of these studies will be directly relevant to new treatments aiming to reduce atherosclerosis by increasing HDL levels.